Fault indicating fluorescent ballast apparatus



vYNW; 1966" A. E. FEINBERG ETAL 3,287,599

FAULT INDICATING FLUORESCENT BALLAST APPARATUS Filed April 6, 1964 Y 2Sheets-Sheet l WHITE 22 VOLT BLACK Nov. 22, 1966 A. E. FEINBERG ETALFAULT INDICATING FLUORESCENT BALLAST APPARATUS 2 Sheets-Sheet 2 FiledApril 6, 1964 United States PatentO 3,287,599 FAULT INDICATINGFLUORESCENT BALLAST APPARATUS Albert E. Feinberg, Chicago, and Fred W.Abrahams,

Skokie, Ill., assignors to Advance Transformer Co.,

Chicago, Ill., a corporation of'lllinois Filed Apr. 6, 1964, Ser. No.357,498 5 Claims. (Cl. 315126) This invention relates generally toballasts for fluorescent lamps systems and more particularly isconcerned with apparatus for indicating the existence of a fault in aballast which is equipped with an automatic resetting thermostat and/ orin the fluorescent lamp system including such ballast.

As well known, the modern fluorescent lamp is a negative resistancecharacteristic device and requires ballasting apparatus to impede theflow of current through the lamp once it has become ignited. The modernfluorescent lamp operates at a voltage substantially higher than that ofthe ordinary power lines available, even in industrial installations,and consequently transformation is required in addition to impedingcurrent flow. By far the greater percentage of fluorescent lamp fixtureshave two or more lamps mounted therein and the apparatus which furnishesthe ballasting effect and transformation is likewise called upon toignite a plurality of lamps and maintain the same in operation.

The fluorescent ballast art has acquired a high degree of sophisticationby virtue of which many ingenious and economical arrangements andsystems are capable of being constructed including ballasts which areeasily obtainable and readily manufactured for use therewith.

Ballasts are known which ignite fluorescent lamps and furnish theoperating voltage therefor, such a ballast hav? ing high leakagereactance built into it so that once the lamps become ignited andcurrent flows through the windings affected by the high leakagereactance, current flow through the lamps is limited. Such a ballast mayignite the lamps one after the other so that a high voltage first willbe applied to one lamp and then to the other, after which the lamps areoperated in series. Other ballasts are known which have parallelcircuits for the lamps.

Some fluorescent lamp systems in use have connections includingso-called disconnect sockets for the lamps. The use of disconnectsockets is a safety measure since this type of socket with which animportant phase of the invention hereinafter will be described has apair of contacts which are open until and unless a terminal of thefluorescent lamp is inserted into the socket to bridge the contacts.Removal of the lamp will open the circuit including the contacts so thatif, for example, the circuit is one in the primary winding of atransformer, the re? moval of the lamp will open the primary energizingcircuit. I

Another aspect of safety which has become more and more important inrecent years is concerned with preventing the ballast itself frombecoming destroyed through a fault or failure in the system, andpreventing the ballast from becoming heated to a point where the pottingcompound therein escapes from the canister of the ballast and causesproperty and personnel damage in addition to the deleterious effects ofescape of the potting compound from the ballast. Safety is also a factorin taking steps to prevent the condenser, usually potted with a ballast,from becoming overheated thereby causing the insulating oil frombreaking down, vaporizing and bursting out of its canister. This, ofcourse, could cause damage to property and personnel as well.

Ballast failure and/or heating up of ballasts may be caused by any oneof many different factors. Several of these factors are faulty wiring,defective lamps, improper 3,287,599 Patented Nov. 22, 1966 connectionsand the like. As a general rule, the fault is first manifest'in anincrease in the heat generated by the ballast, and since the windingwhich normally has the highest current flowing therein is the primarywinding, the heat normally will be greatest in the vicinity of theprimary winding. One fault which especially produces this heating oftheprimary winding due to its drawing a greater than normal current islamp rectification. Some fluorescent lamps, such as the so-calledslimline or instant start lamps, act as rectifiers when one or both endsof the lamp become deactivated.

It has become common, therefore, to connect a fusible link or otherdisconnect means operated by high temperature in the primary circuit,such means being in the immediate vicinity of the primary winding.Fusible links render the ballast useless and hence in many ballasts anautomatic reset thermostat is connected in series with the primarywinding operable by an increase of temperature in the vicinity of theprimary winding. Such a thermostat is advantageous where the cause offault is external of the ballast so that once the fault has been cured,the ballast can operate satisfactorily.

An automatically resetting thermostat would be one which includes abimetal member that is engaged with a contact at normal temperatures butswings away from the contact at abnormal temperatures. The arm havingthe bimetal and the contact are in series in the primary circui't sothat when the bimetal member moves from contact, there will no longer beanyvcurrent flowing in the primary. The primary, therefore, will cooland lower the temperature in its vicinity whereupon the bimetal willswing back to the contact and once more complete the circuit. I

The user will recognize that a fault has occurred because thefluorescent lamps of the particular fixture will both 'go out if thereare two lamps in the fixture. After a while the lamps will light oncemore since the cooling of the primary winding has permitted the bimetalmember to return to its normal position engaging the contact of thethermostat. This cycle may repeat indefinitely if the temperatures arenot too severe but even this arrangement may not save the ballast.

For the purposes of this discussion, it may be assumed that the userhaving seen the de-energizing of the fluorescent lamps realizes thatthere is a fault and will take steps to cure it.

In most installations the arrangement described is statisfactory but ina great many instances the fixture in which the fault occurs may be oneof many. Two effects may occur. The recycling may occur a great manytimes with a gradual deterioration of the ballast, especially itscondenser, so that finally the ballast fails at a fairly lowtemperature. may have to wait a considerable length of time betweencycling to choose which one of a large installation of fixtures is thedefective one. The time lost can be substantial where the cycling timeof the thermostat is made relatively long to assure protection of theballast, thermostat and lamps.

It is, therefore, an important object of the invention to provideapparatus for use with a ballast for fluorescent lamps having anautomatic reset thermostat in circuit with the ballast which apparatuswill deactivate the fixture once the thermostat operates and keep thefixture deactivated and incapable of being further activated withoutfirst completely disconnecting the power from the fixture.

A further object of the invention is to provide apparatus for indicatingthe fault in a fluorescent ballast without the need for expensive orcomplicated apparatus and without radically altering the wiring ofthesystem and ballast.

The second factor is that a maintenance man Still a further object ofthe invention is to provide apparatus of the character described inwhich once a fault has occurred in a ballast and the ballast has beendeactivated by an automatic reset thermostat built into the ballast,'theballast remains deactivated even after the ballast has cooled oif andthe thermostat has reset itself, the ballast remaining in thisdeactivated condition until the ballast has been electricallydisconnected from the line.

In connection wit-h the object set forth immediately above, it is afurther object of the invention to provide a fluorescent ballast andlamp system in which after the ballast and its fluorescent lamps havebeen deactivated due to a fault and remain deactivated in accordancewith the invention, the removal of a lamp from the fixture by themaintenance man or user will give control of the apparatus back to thethermostat so that upon reinsertion of an operative lamp in the fixturethe entire fixture and ballast will be reactivated.

There are many other objects and advantages to this invention which willappear from a careful reading and examination of the description whichfollows, this description being the detailed disclosure required bystatute directed to specific apparatus offered as preferred embodimentsof the invention. In connection with this disclosure, the drawingillustrates the same diagrammatically and symbolically.

In the said drawing:

FIG. 1 is a combined circuit diagram .and symbolic representationillustrating apparatus instructed in accord ance with the invention, theballast and system the-rein depicted being known as a lead-lag ballastand fluorescent lamp system.

FIG. 2 is a circuit diagram of the apparatus of FIG. 1 but in a moresimplified form.

FIG. 3 is a circuit diagram of a fluorescent ballast and lamp system ofthe so-called series-sequence type having the invention associatedtherewith.

FIG. 4 is a circuit diagram of a fluorescent lamp ballast and system ofthe so-called rapid start variety in which a modified form of theinvention has been embodied.

Generally, the invention is characterized by the provision of a relay ofsimple construction connected in a manner such as to be in series withthe primary circuit when the solenoid of the relay is energized, therebyproviding a by-p-ass circuit around the automatic thermostat. The relayis parallel with the thermostat and hence will automatically operatewhen the thermostat opens, withdrawing control from the thermostat. Onlyone additional lead is required, this being connected from that side ofthe thermostat which is connected to the primary winding to the contactof the relay that extends to the line. The relay contacts are insertedinto the connection from the automatic thermostat to the same line sothat when the relay solenoid is not energized, the bridging member ofthe relay closes the contacts but as soon as the solenoid is energized,those contacts are open and will remain open so long as current flowsthrough the solenoid.

The circuit with which the solenoid and thermostat are associated ispreferably one which includes so-called disconnect sockets, each lamphaving one of its terminals engaged in such a socket so that the removalof the lamp will open the line in which the sockets are connected. Inaccordance with standard practice these disconnect sockets are also inseries with the primary winding. Removal of a lamp will therefore openthe primary winding. If the ballast and lamps have been deactivated byoperation of the relay, the removal of a lamp from its socket willdeenergize the relay solenoid turning the control of the apparatus backto the thermostat by virtue of the closing of the contacts of the relay.

As will be seen from the description below, in such circuits there is noneed to provide a manual reset on the relay. In other circuits wherethere are no disconnect sockets, all that there need be is a simpleswitch anywhere in the primary circuit, to open that circuit forde-ener- 30 through the lead 4 gizing the relay solenoid to restorecontrol to the thermostat. The thermostat has, of course, cooled off inthe meantime, and reset itself.

The basic concept of the invention is that of providing a relativelyhigh impedance by-pass circuit having a relay in parallel with theautomatic reset thermostat which (a) keeps the thermostat connectionopen, (b) remains energized so long as the power line is connected tothe ballast, and (c) absorbs the greater portion of the total voltage inthe series circuit including the primary winding .and relay solenoid sothat the current in the circuit is substantially less than the minimummagnetizing current which would energize the ballast. As a result, thefluorescent lamps are extinguished and there is no detectable heatingelfect in the ballast, so that the faulty fixture containing ballast andlamps is readily located.

Looking now at the drawing, in FIG. 1 the reference character 10designates a ballast and fluorescent lamp system which comprises theballast 12 and the two fluorescent lamps 14 and 16 connected together.This ballast and fluorescent lamp system is adapted to be energized froma suitable source of power such as a 120 volt A.C. line at the terminals18 and 20 by means of leads 22 and 24 respectively extending to suchterminals. Preferably the lead 22 is at ground potential and hence isdesig-.

nated white in FIG. 1.

The ballast 12 is substantially of conventional construction and in thiscase is of the so-called lead-lag variety. There is an elongate ironcore 26 having a central winding leg 28 upon which are mounted a primarywinding 30 and two secondary windings 32 and 34 at opposite ends of thecentral winding leg 28. The exact construction of the code is immaterialto the invention although for the particular arrangement described,there will be shunts at 36 and 38 to result in a relatively high leakagereactance in the windings 32 and 34 during operation of the ballast.

The core 26 and the windings described comprise a transformer designated40 that is preassembled and immersed in a potting compound 42 ofasphaltic or similar nature contained in a steel canister 44 along witha condenser 46 of the oil immersed variety.

As previously mentioned, since the highest temperature achieved in theballast will probably be in the vicinity of the primary winding 30, anautomatic reset thermostat 48 is positioned adjacent the winding 30 ontop of the.

iron core 26 if desired.

The secondary 32 is the lag secondary and it has one.

of its terminals connected by the lead 50 to the contact 52 engaged bythe pin 54 of the lamp 16. The other terminal of the secondary winding32 connects by way of the lead 56 to the junction 58. This junction 58is a point common with a terminal of the primary winding 60, theterminal of the lead secondary 34 through the lead 62 and one side ofthe thermostat 48 through the lead 64. The primary winding 30 isenergized by way of the thermostat 48 and the lead 66 through thecontacts 68 and 70 and the bridging bar 72 of the relay 74 which will bedescribed. Terminal 70 connects to the conductor 24. The other terminalof the primary winding 30 connects by way of the lead 76 through theopposed contacts of two disconnect sockets 78 and 80 to the lead 22.Obviously the pins 82 and 84, being the left-hand terminal pins of thelamps 14 and 16 respectively, must be in place in the respectivedisconnect sockets 78 and 80 in order that the primary winding 30 beenergized.

The lead secondary winding 34 has a terminal connected by way of thelead 86 to one side of the condenser 46, the other side of the condenserbeing connected by the lead 88 to the contact 90 which isengaged by thepin 92 of the lamp 14.

From the above description, it will be apparent to those skilled in theart that except for the relay 74 and its contacts and bridging bar, thecircuit is well-known, even to the inclusion of the disconnect sockets.In operation, each of the secondary windings is in autotransformerrelationship with the primary winding 30 and therefore, the circuits ofthe respective lamps 14 and 16 ignite at about the same time and operatein parallel at 21 voltage reduced from the igniting voltage because ofthe high leakage reactance built into the transformer 40. The effect ofthe condenser 46 is primarily for power factor correction although somestroboscopic correction is achieved.

As shown in FIG. 2, which is a simplified circuit diagram of FIG. 1, theautomatic reset thremostat 48 has a contact 100 and a bimetal member 102that is normally engaged against the contact 100 to close the circuitfrom the juncture 58 through the lead 66 of the contact 68 bridging bar72, contact 70 and the lead 24 to line terminal 20. When the bimetalmember 102 is subjected to heat of abnormal degree, it commences to moveoff the contact 100 and when it does, it will open the lead 66 therebyeffectively de-energizing the primary winding 30 as well as the ballastwindings 32 and 34 and the two lamps 14 and 16. As soon as magnetizingcurrent stops flowing in the primary winding 30 (disregarding the actionof the relay 74 for the time being), the ballast starts to cool andafter a time, depending upon the physical characteristics of thethermostat 48, the bimetal member 102 will return to the contact 100 andestablish an electrical connection and the cycle begins again.

The simple relay 74 prevents the restoration of the bimetal arm 102 tothe contact 100 from activating the ballast and circuit. An electricalconnection in the form of the lea-d 104 extends from the juncture 58 inseries with the solenoid 106 to the contact 70, bypassing the thermostat48, the lead 66, the contact 68 and the bridging bar 72. It is importantto note that this bypass circuit is permanenly connected and thereforethe relay 74 is fully automatic in operation. With the thermostat 48closed, no current flows in the bypass circuit 104 and the solenoid (106is not energized. The bridging bar 72 engages the contacts 68 and 70.All of the current passing through the primary winding 30 will passthrough the thermostat 48 and the lead 66 since there is no impedance inthat circuit and the current prefers it to the impedance of the solenoid106. The primary winding draws the maximum of magnetizing current atthis time, limited only by its own impedance. When, however, a faultoccurs such as rectification in one or the other of the lamps 14 and 16,causing the ballast 12 to heat up, the thermostat 48 will open. At thismoment, current now flows through the bypass circuit. The solenoid 106of the relay 74 is energized by this current and it pulls the bridgingbar 72 away from the contacts 70 and 68 but remains in this conditionsince the solenoid is connected directly to the contact 70 whichconnects to the line. Tracing the circuit, it will be seen that itextends from the juncture 58 through the primary Winding 30 and the twodisconnect sockets 80 and 78, the conductor 22 and back to the otherside of the line 18. From this point forward, therefore, so long asnothing occurs in a circuit, and providing the lamps 14 and 16 remain inplace, the only current flowing through the ballast is that which isdrawn due to the impedance of the solenoid 106 and the primary 30.

The impedance of the solenoid 106 is much greater than the primaryimpedance on practically all ballasts which would have a thermostat.This could be of the order of ten times, so that for a ballast normallydrawing 500 milliamperes, the flow of current is limited to somethingless than 50 milliamperes. The low current in the primary winding willnot materially raise the temperature of the primary winding nor will itbe sufiicient to ignite or operate the lamps. Most of the potential dropin the series circuit described including the solenoid 106 and thewinding 30, will occur in the solenoid 106.

Assuming the apparatus 10 is a part of a large installation, amaintenance man may merely look at the installation and see that one ofthe fixtures is not operating since its lamps are not illuminated. Hetherefore inspects and repairs the apparatus. During the course of doingso, or as a part of such activity, he will remove the lamps 14 and 16and replace them or reinstall them. As soon as one of these lamps isremoved, it breaks the circuit including the bypass therebyde-energizing the solenoid 106 and reconnecting the thermostat 48 to theline. In the meantime, the thermostat 48 will have cooled and restoredits connection but such restoration will have no effect upon the ballastso long as the solenoid 106 is energized.

The circuit diagram in FIG. 3 illustrates a ballast and fluorescent lampsystem 110 which is known as a seriessequence circuit. The constructionand operation of the basic circuit is disclosed in US. Patent 2,558,293.In the apparatus, which uses instant start lamps similar to those ofFIGS. 1 and 2, there is a transformer 112 which has its primary winding114 and a high leakage reactance secondary winding 116 connected inseries with a condenser 118 and a lamp 120. When the primary winding 114is energized from the line terminals 122 and 123, there will besufficient voltage developed in the windings 114 and 116 to ignite thelamp 120. Current thereafter flows through the lamp and the condenser118, as a result of which there is a phase reversal in the secondarywinding 116. The windings 114 and 116 are in additive relationship onopen circuit and windings 116 and 126 are bucking during open circuit.When, however, the winding 116 carries the current resulting from theignition and operation of the lamp 120, there is a voltage in thatwinding which is now additive to that of 126 and the resultant voltageacross the lamp 128 is sutficient to ignite that lamp. Prior to thistime the lamp 128 did not have suflicient voltage.

During the operation, since the winding 116 develops a very high leakagereactance therein, current prefers to flow in series through the twolamps, the condenser 118 and the secondary winding 126.

As shown herein, the left-hand terminal 130 of the lamp 120 is engagedin the disconnect socket 132 and from there current must flow throughthe lead 134 through the thermostat 136 of the lead 138, the contacts140 and 142 of the relay 144 to theline terminal 122 through a lead 146.The bridging bar 148 normally connects the contacts 140 and 142, thisbeing a normally closed relay as is the relay 74 in FIGS. 1 and 2. i

There is another disconnect socket 150 engaged by the left-hand pin 152of the lamp 128.

The bypass circuit in this case is connected from the lead 134 through aconductor 154 to a solenoid 156 of H the relay 144 to the contact 142.

The operation of this apparatus is quite similar with respect to themanner in which a fault will open the thermostat 136 thereby causing thebar 148 to be pulled off the contact 140 and 142. The condition remainsuntil one or the other or both of the lamps 120 and 128 are removed fromtheir respective disconnect sockets 132 and 150. Since the thermostat136 resets itself because of cooling of the primary winding 114, as soonas the circuit in series with the primary winding opens because of theremoval of one or more lamps, the apparatus 110 is ready to be operatedonce more when the lamps are returned to their sockets.

In FIG. 4, there is illustrated a circuit typical of those which do notuse disconnect sockets such that the completely automatic operation ofthe circuit cannot be achieveed in accordance with the invention.Illustrated is apparatus 160 in the form of a rapid start fluorescentlamp ballast and the circuit. The ballast in this case includes atransformer 162 having a primary winding 164 and a secondary winding 166connected in autotransformer relationship with a pair of rapid startlamps 170 and 172. It will be appreciated that these lamps are inseries. The apparatus is energized from a source of power consisting ofconventional 120 volt line, the terminals being shown at 174 and 176.The starting condenser 178 is connected across the lamp 172 whereas thepower factor condenser 180 is connected to the right-hand terminal ofthe winding 166 through the lead 182.

Each lamp has filaments at opposite ends thereof and filament windingsare provided as shown at 184. The connection of the filament windingsand the circuit are familiar.

As shown, the primary winding 164 has one lead 186 which extends to theterminal 176 but the other terminal connects by way of the lead 188through an automatic reset thermostat 190 to the lead 192 which extendsthe connection through the contacts and cross-bar (not shown) of therelay 194 to the lead 196 to the terminal 174. The construction of therelay 194 is substantially the same as relays 144 and 74. The bypasscircuit around the thermostat 190 in this case consists of the lead 198which like the other relays described is in series with the solenoid ofthe relay 194 and connects to the hot terminal as shown by theconnection to the lead 196.

There is a manual disconnect switch 200 in the lead 198 which serves thepurpose of de-energizing the solenoid after the thermostat 190 is open,for giving control back to the thermostat 190. When a fault occurs inthe ballast such as to heat the same, as soon as the thermostat 190opens, relay 194 will take over control and current will flow throughthe relay keeping its contacts open. In the case of circuits of theinvention including the disconnect sockets, the removal of the lamp willserve to de-energize the solenoid but in a circuit such as described inFIG. 4, it is necessary manually to open the bypass circuit. For thisreason, there is a manual disconnect switch 200. When the user is readyto set the apparatus into operation once more, he presses the button oroperates the switch of the manual disconnect 200 thereby opening thelead 198. Since the ballast has cooled in the meantime and thethermostat 190 is now closed, it takes over controls so that current maynow flow directly from the line terminals 174 and 176 through theprimary winding 164 when the lamps are in place. j As pointed out, thecircuit of FIG. 4 does not have all of the advantages of those of FIGS.1, 2 and 3, but it serves the purpose of locating the fault before themaintenance man and prevents further operation and possible damage ofthe ballast once the thermostat 190 has operated.

Other variations are capable of being made within the scope of theinvention as defined by the appended claims. What it is desired tosecure Letters Patent of the United States is:

1. A fluorescent lamp and ballast system comprising: a transformerhaving a primary winding and at least one secondary winding inductivelycoupled to the primary winding, at least one fluorescent lamp connectedin circuit with the secondary winding to be ignited and operated by thetransformer, leads extending from two terminals of the primary windingand adapted to be connected respectively to two terminals of an A.C.power source, and

one lead having an automatic reset thermostatic switch series connectedtherein and disposed adjacent said transformer to be sujected to theheat generated therein so that abnormal heat from said transformer willopen said thermostatic switch, with respect to a fixed contact thereof,at a predetermined temperature and thereby open said lead to saidprimary winding to deenergize said transformer andextinguish said lamps,following which the switch will close after the transformer has cooledto a second lower temperature once more closing said lead to saidprimary winding energizing said transformer and reigniting said lampunless otherwise prevented from doing so, a relay having normally closedcontacts in series with said switch in said one lead and the relayhaving a solenoid connected in a circuit bypassing the switch andcontacts, the solenoid impedance being substantially higher than theimpedance of the primary winding whereby upon opening of said switchcurrent will flow in said solenoid in series with said primary winding,the solenoid being mechanically connected to open said contacts whenenergized, and the solenoid adapted to draw a current smaller thanrequired to energize the transformer but sufficient to keep the solenoidenergized so that the subsequent closing of the switch will not alfectthe primary winding until the solenoid is deenergized and means fordeenergizing the solenoid comprising circuit interrupting means in theseries circuit of the primary winding and solenoid.

2. A circuit as claimed in claim 1 in which said circuit interruptingmeans comprises a second manually operable switch.

3. A circuit as claimed in claim 1 in which said circuit interruptingmeans comprises a disconnect socket and said lamp has a terminal thereofengaged in said socket so that removal of said lamp from said socketwill deenergize said solenoid.

4. A circuit as claimed in claim 1 in which there is a pair of secondarywindings and a pair of fluorescent lamps and each lamp is connected inseries with one secondary winding, respectively, there being a condenserin series with one lamp.

5. A circuit as claimed in claim 1 in which there are two secondarywindings and a condenser, one secondary winding and condenser beingconnected in a loop with one lamp, and the secondaries both beingconnected in a second loop with another lamp, the secondaries bucking anopen circuit and the one secondary being loosely cou: pled to theprimary winding.

References Cited by the Examiner UNITED STATES PATENTS 2,146,498 2/1939Bradley 315- 2,305,096 12/1942 McDermott 3l5-127 X 2,309,711 2/1943Pearson 3 l5127 2,534,107 12/1950 Curzon 200-88 X 2,560,771 7/1951Lamont 315l22 JOHN W. HUCKERT, Primary Examiner. D. O. KRAFT, AssistantExaminer.

1. A FLUORESCENT LAMP AND BALLAST SYSTEM COMPRISING: A TRANSFORMERHAVING A PRIMARY WINDING AND AT LEAST ONE SECONDARY WINDING INDUCTIVELYCOUPLED TO THE PRIMARY WINDING, AT LEAST ONE FLUORESCENT LAMP CONNECTEDIN CIRCUIT WITH THE SECONDARY WINDING TO BE IGNITED AND OPERATED BY THETRANSFORMER, LEADS EXTENDING FROM TWO TERMINALS OF THE PRIMARY WINDINGAND ADAPTED TO BE CONNECTED RESPECTIVELY TO TWO TERMINALS OF AN A.C.POWER SOURCE, AND ONE LEAD HAVING AN AUTOMATIC RESET THERMOSTATIC SWITCHSERIES CONNECTED THEREIN AND DISPOSED ADJACENT SAID TRANSFORMER TO BESUBJECTED TO THE HEAT GENERATED THEREIN SO THAT ABNORMAL HEAT FROM SAIDTRANSFORMER WILL OPEN SAID THERMOSTATIC SWITCH, WITH RESPECT TO A FIXEDCONTACT THEREOF, AT A PREDETERMINED TEMPERATURE AND THEREBY OPEN SAIDLEAD TO SAID PRIMARY WINDING TO DEENERGIZE SAID TRANSFORMER ANDEXTINGUISH SAID LAMPS, FOLLOWING WHICH THE SWITCH WILL CLOSE AFTER THETRANSFORMER HAS COOLED TO A SECOND LOWER TEMPERATURE ONCE MORE CLOSINGSAID LEAD TO SAID PRIMARY WINDING ENERGIZING SAID TRANSFORMER ANDREIGNITING SAID LAMP UNLESS OTHERWISE PREVENTED FROM DOING SO, A RELAYHAVING NORMALLY CLOSED CONTACTS IN SERIES WITH SAID SWITCH IN SAID ONELEAD AND THE RELAY HAVING A SOLENOID CONNECTED IN A CIRCUIT BY-PASSINGTHE SWITCH AND CONTACTS, THE SOLENOID IMPEDANCE BEING SUBSTANTIALLYHIGHER THAN THE IMPEDANCE OF THE PRIMARY WINDING WHEREBY UPON OPENING OFSAID SWITCH CURRENT WILL FLOW IN SAID SOLENOID IN SERIES WITH SAIDPRIMARY WINDING, THE SOLENOID BEING MECHANICALLY CONNECTED TO OPEN SAIDCONTACTS WHEN ENERGIZED, AND THE SOLENOID ADAPTED TO DRAW A CURRENTSMALLER THAN REQUIRED TO ENERGIZE THE TRANSFORMER BUT SUFFICIENT TO KEEPTHE SOLENOID ENERGIZED SO THAT THE SUBSEQUENT CLOSING OF THE SWITCH WILLNOT AFFECT THE PRIMARY WINDING UNTIL THE SOLENOID IS DEENERGIZED ANDMEANS FOR DEENERGIZING THE SOLENOID COMPRISING CIRCUIT INTERRUPTINGMEANS IN THE SERIES CIRCUIT OF THE PRIMARY WINDING AND SOLENOID.